Simultaneous development-cleaning of the same area of an electrostatographic image support surface

ABSTRACT

A system for removing residual toner images from electrostatographic image support surfaces and simultaneously developing an undeveloped electrostatic latent image on essentially the same area of said surface, including developmentcleaning a xerographic plate, for example, by cascading developer along the image support surface of the plate.

United States Patent Cade et al.

[ 1 Mar. 14, 1972 [54] SIMULTANEOUS DEVELOPMENT- CLEANING OF THE SAME AREA OF AN ELECTROSTATOGRAPHIC IMAGE SUPPORT SURFACE [72] Inventors: Ronald L. Cade, Fairport; Stewart William Volkers, Williamson, both of N.Y.

[73] Assignee: Xerox Corporation, Rochester, NY.

[22] Filed: Dec. 31, 1968 [21] Appl. No.: 789,031

[56] References Cited UNITED STATES PATENTS 3,424,615 1/1969 Eichometal. ..134/7 3,520,604 7/1970 Shelffo ..355/16 2,573,881 11/1951 Walkup et al.7 117/17.5 X 2,756,676 7/1956 Steinhilper. 117/ 1 7.5 X 2,874,064 2/1959 Andrus ..117/17 5 2,880,699 4/ 1959 l-layford ..1 17/17.5 X 2,911,944 11/1959 l-layford et a1... ..118/637 2,956,487 10/1960 Giaimo 1 17/17.5 X 2,959,153 11/1960 Hider..... ..118/637 3,008,826 11/1961 Mott et al. ...117/17.5 X

3,157,546 11/1964 Cover ..117/17.5 X

3,257,223 6/1966 King ..118/637 X 3,412,710 ll/l968 Robinson..... 117/17.5 X 3,424,131 1/1969 Aser et al ..96/l.4 X 3,472,657 10/1969 Mayer et al. 117/17.5 X 3,484,265 12/1969 Swyler ..l17/l7.5 3,503,776 3/1970 Gundlach ..117/17.5

Primary Examiner-William D. Martin Assistant Examiner-Edward J. Cabic Att0meylames J. Ralabate, Roger W. Parkhurst and David C. Petre [57] ABSTRACT A system for removing residual toner images from electrostatographic image support surfaces and simultaneously developing an undeveloped electrostatic latent image on essentially the same area of said surface, including developmentcleaning a xerographic plate, for example, by cascading developer along the image support surface of the plate.

25 Claims, 3 Drawing Figures PATENTEDHAR 14 I972 I 3, 649,262

SHEET 1 BF 2 INVENTORS STEWART WILLIAM VOLKERS RONALD L. CADE 8y QW Q MM ATTORNEY PATENTEDMAR 14 I972 I 3,649,262

sum 2 BF 2 VERY HIGH NS S ARE NOT C NE 600- MINIMUM INPUT/ s' MAXIMUM INPUT SITY ADEQUATELY ANED CHARGING VOLTAGE OPERATING RANGE 200- Y LOW DENSI NOT RE PRODU D O l I l l f/ll.3 f/8 6 V55 v4.5

RELATIVE EXPOSURE ILLUMINATION SIMULTANEOUS DEVELOPMENT-CLEANING OF THE SAME AREA OF AN ELECTROSTATOGRAPHIC IMAGE SUPPORT SURFACE BACKGROUND OF THE INVENTION This invention relates to electrostatic imaging and more specifically to the development of electrostatic latent images and the removal of the residual toner images from a. support surface.

The most successful electrostatic imaging process and one preferred in the present invention is that of xerography.

Xerography was first described in Carlson U.S. Pat. No.

2,297,691. Generally, the xerographic process is performed upon a xerographic plate comprising a layer of photoconductive insulating material upon a conductive backing. The surface of the plate is uniformly charged and then exposed to a light and shadow image pattern. The photoconductive plate discharges in the exposed areas proportionally to the intensity of the radiation reaching the exposed area, thereby creating an electrostatic latent image on the surface of the photocon ductive layer corresponding to the light and shadow image pattern projected upon the plate. The electrostatic latent image is then developed by contact with an electroscopic marking material called toner. The electrostatic latent image which has been developed by contact with toner is then referred to as the toner image or developed image". This developed image may be fixed on the xerographic plate itself, or it may be transferred to paper or other material, and the transferred image may be fixed on said other material. However, after the developed image is transferred to another base material, there may still be and there typically is, a residual image of toner particles adhering to the surface of the photoconductive layer. if this residual image is not removed before the plate is reused, portions of the residual image may be transferred and fixed to any new copy which is made from the same plate.

Because of the reusable nature of particular photoconductive insulating materials such as those comprising amorphous selenium, it has been possible to automate the xerographic process for example using a rotating xerographic plate in the form of a cylindrical drum. One such automatic recyclable apparatus is described in U.S. Pat. No. 3,062,109. One of the most successful of such automated embodiments of the xerographic process has been the 914 copier, manufactured by the Xerox Corporation, Rochester, New York. In such xerographic apparatus, the process steps are performed at various stations located around the periphery of the drum. These stations generally perform the typical process steps: charge, expose, develop, transfer, and clean, in said sequence.

To control and prohibit subsequent transfer of residual images in such automatic apparatus, it has been found to be commercially expedient to develop and clean in sequence and at separate locations around the rotating drum xerographic plate and to charge the plate to a polarity typically opposite that of the polarity of the preexposure charge after the toner image is transferred and before the plate is cleaned and recycled. Charging a xerographic plate after transfer with an oppositely charging corona allows the residual toner particles to be more readily brushed off the surface by any suitable means such as a rotating fur brush. Once the surface of the xerographic plate is cleaned, the entire xerographic cycle may be started again on the same plate.

Although the flat rigid plate configuration and the drum configuration of the xerographic plate are the most widely used in commercial xerographic processing, the plate may take any suitable form including a web, foil, laminate or the like, metallic strip, sheet, coil, cylinder, drum, endless belt, endless mobius strip, circular disc or other shape. In addition, the electrically conductive support may comprise two or more layers depending upon the desired characteristics of the support plate as a whole. The words xerographic plate or plate are commonly used herein to designate any of these various configurations. In each case, the form of the plate surface may control the manner in which the various xerographic process steps may be performed upon the plate.

Similarly, electrostatic latent images may be formed by methods in addition to the preferred mode of charging and exposing a xerographic plate. Other modes include charging or sensitizing in an image configuration through the use of a mask or stencil, or by first forming such a charge pattern on a separate photoconductive insulating layer according to conventional xerographic reproduction techniques and then transferring this charged pattern to the surface of another plate by bringing the two into very close proximity and utilizing breakdown techniques as described, for example, in Carlson U.S. Pat. No. 2,982,647, and Walkup U.S. Pat. Nos. 2,825,814 and 2,937,943. In addition, charge patterns conforming to selected shaped electrodes or combinations of electrodes may be formed on a support surface by the 'lESl" discharge technique, as more fully described in Schwertz Pat. Nos. 3,023,731 and 2,919,967, or by techniques described in Walkup Pat. Nos. 3,001,848 and 3,001,849, as well as by electron beam recording techniques, as described in Glenn U.S. Pat. No. 3,113,179.

Although xerography isv here described as the presently preferred process in which the present invention may be incorporated, it will be seen that the advantageous system of the present invention may be incorporated in any sort of reuseable electrostatographic process. Electrostatography is defined as the formation and utilization of latent electrostatic charge patterns for the purpose of recording and reproducing patterns in viewable form (See Standard Definitions of Terms for Electrostatographic Devices, lEEE No. 224, Nov. 1965, published by The Institute of Electrical and Electronics Engineers, Inc., 345 East 47 Street, New York, N.Y. 10017.)

Referring now more specifically to the development and cleaning steps, a commercially successful mode of develop ment employed in automatic xerographic apparatus is described in Walkup U.S. Pat. No. 2,618,551, wherein a developer generally consisting of toner and a granular material called carrier, which by mixing triboelectrically acquire charges of opposite polarity, is gravitationally cascaded over the xerographic plate carrying the electrostatic latent image. Although carrier typically comprises spherical particles, in various other systems the carrier may be in various forms and substances including flat platelets, cubical solids, synthetic and natural fibers, metallic filings, and others. In addition to the cascade development system, magnetic brush, liquid developer, fluidized bed, powder cloud and other development systems are well known.

A commercially successful mode of cleaning employed in automatic xerographic apparatus is described in U.S. Pat. Nos. 2,751,616 and 2,832,977, wherein a brush with bristles which are soft and of suitable triboelectric characteristics, and yet sufiiciently firm to remove residual toner particles from the xerographic plate, is used to whisk residual toner particles from the xerographic plate. In addition, webs or belts of soft fibrous materials or tacky materials, and other cleaning systems are known.

In spite of the successes that have been achieved in cleaning, the prior art solutions to the problems in the development and cleaning steps in the xerographic process are not entirely satisfactory. For example, cleaning still typically requires bulky apparatus and a separate and distinct cleaning station. Experience has shown that the greater the number of apparatus stations necessary to carry out the xerographic process, the greater the danger of toner powder escaping throughout the mechanism and dusting the operating apparatus. Many cleaning systems typically require more than one pass through the cleaning station, requiring more time for cleaning the xerographic plate and thereby making the cleaning step one of the limiting factors in the operating time of the xerographic cycle. Also, typically development and cleaning must be performed at different areas of the xerographic plate, which requires more apparatus to ensure that that portion of the xerographic plate being used to reproduce the desired image is correctly registered at each of the xerographic stations. Experience in the art of photoconductors has shown that the greater the number of passes necessary to clean or develop the surface of said photoconductor, the fewer the number of cycles through which said photoconductor or xerographic plate can be used with acceptable image quality. The surface of the photoconductor is partially abraded by multiple passes through development or cleaning steps, and scratches in the surface of the plate may mechanically pick up toner particles thereby darkening the background areas of desired images. In addition, increased numbers of passes through development or cleaning stations tend to increase toner consumption and to impair toner concentration in the developer system. Each of these effects contributes to reduced image quality in the prior art systems.

Thus it is seen that even the most successful and advanced xerographic apparatus includes bulky cleaning apparatus at a separate station, which tends to increase toner consumption and toner dusting of the apparatus, detracts from the useful life of the photoconductor, and after extended use reduces copy quality.

In the past, attempts have been made to overcome the various disadvantages just enumerated, but none of the proposed solutions appear to be as advantageous as the surprising system of the present invention. For instance, Copley US. Pat, No. 2,484,782, teaches cascading granular material alone, charged oppositely to toner particles over the part of the xerographic plate containing the residual toner image to remove said image. But this system still requires a separate process station with all of its attendant disadvantages. Done]- son et al. US. Pat. No. 3,146,687 describes a system wherein various sectors of a circular plate are sequentially passed through xerographic steps including cleaning and development. But this system requires multiple pass cleaning and special mechanical apparatus to ensure that the desired sector of the circular xerographic plate is properly registered at each of the process stations. Howell US. Pat. No. 3,108,895, erases dielectric recording media having an electrically conductive backing layer by drawing said media through a mass of electrically conductive particulate ink, said mass being in electrical connection with the conductive backing of the dielectric media. This system again requires multiple pass cleaning in a separate cleaning station.

Thus, there is a continuing need for better systems for the development of electrostatic latent images on a support surface such as the surface of a xerographic plate, and for systems for cleaning residual toner images from such surfaces.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide an imaging system which overcomes the above-noted disadvantages and satisfies the above noted needs.

It is also an object of this invention to provide a system for development of latent images and removal of residual images on a support surface.

It is another object of this invention to provide a system for the simultaneous development and cleaning of an electrostatic latent image support surface or electrostatographic surface.

It is another object of this invention to provide a system for the simultaneous development and cleaning of essentially the same area of an electrostatographic surface such as a xerographic plate.

It is another object of this invention to provide a system for the simultaneous development and cleaning at essentially the same area of a xerographic plate at the same station in a xerographic apparatus.

It is another object of this invention to provide a system for cleaning the surface of a xerographic plate in a single cleaning pass.

It is another object of this invention to provide a system for improving image quality of copies made by the xerographic process.

It is yet another object of this invention to provide a system to increase toner efiiciency in the xerographic process.

It is another object of this invention to provide a system to prolong the life of the photoconductive insulating layer of the xerographic plate, or any other exposed surface of the xerographic plate or support surface.

It is still another object of this invention to provide a system enabling more quiet operation of electrostatic copying apparatus.

It is another object of this invention to provide a system for the reduction of the amount of toner escaping from and thereby contaminating and impairing the operation of the various parts of electrostatographic copying apparatus.

It is still another object of this invention to provide a more simple and compact electrostatographic copying apparatus.

The foregoing objects and others are accomplished in accordance with this invention by providing a system, which when used in conjunction with an electrostatic latent image support surface provides for the substantially simultaneous removal of residual images comprising residual toner particles from said surface, and development of electrostatic latent images or charge patterns on essentially the same area of said surface.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following detailed disclosure of the preferred embodiments of this invention taken in conjunction with the accompanying drawings thereof, wherein:

FIG. l'is a side view of an otherwise typical xerographic apparatus employing the advantageous system of this invention.

FIG. 2 is a side view of a preferred cascade developmentcleaning apparatus used in the embodiment of FIG. 1.

FIG. 3 is a graphic illustration of the preferred operating range of the present invention in one preferred mode of forming the electrostatic latent image, expressed in the variables Charging Voltage and Relative Exposure Illumination used to form said electrostatic latent image.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 which discloses a xerographic apparatus showing the steps typically used in the xerographic process, but embodying the advantageous system of the present invention, 10 designates the rotating drum with photoconductor layer. As the surface 11 of the drum advances during a xerographic cycle, a corona discharge device 12 initially charges said surface. The charged surface then advances through station 13 where the light and shadow image desired to be copied is projected onto the surface 1 l of the drum 10.

The charged and exposed surface of the drum now bearing the electrostatic latent image corresponding to the light and shadow image projected thereon, then advances into the advantageous combination development-cleaning station 14 of this invention. At 14, the advantageous and surprising development-cleaning system of the present invention is carried out by a cascade of developer comprising toner and carrier which develops electrostatic latent images and at the same time, as will be further described, removes residual images comprising toner particles, typically adhering to the surface of the drum in essentially the same area.

The area of the surface just cleaned and developed by the advantageous system of the present invention, and now sup porting the developed image, then preferably, though not necessarily in all embodiments, advances through the field of pretransfer electrode 15 which recharges the surface of the drum in preparation for the transfer step. The surface carrying the developed image next advances into the transfer station 16 where the developed image is transferred to another backing 19. As the paper 20 advances through the fixing process, the corresponding portion of the surface of the xerographic drum from which the image now supported on the paper was transferred, continues to advance through the cycle, but now supporting only the residual image of toner particles remaining after the transfer step. In the apparatus of FIG. 1 after transfer 16, the drum surface advances past a negative charging apparatus 21 wherein the charge on the drum is reversed in preparation for cleaning. 22 designates the position where cleaning apparatus would typically be located in prior xerographic systems. The surface supporting the residual image continues to advance through discharge station 23 where the entire surface of the drum is flooded with light to discharge the photoconductive insulating layer.

After discharge at 23 the surface of the drum is then ready to be charged and exposed again, although the residual image from the previous exposure typically still remains on the same area of said surface. This is possible because the surprising and advantageous system of the present invention is used in the illustrated embodiment. In the previously known xerographic systems, the drum surface would typically require an additional cleaning step before the charging and exposing steps of the subsequent xerographic cycle are performed.

Cascading developer, comprising toner and carrier, along the surface of the photoconductive layer supporting residual toner images, is a preferred mode of development-cleaning. As the developer cascades along the surface of the plate, electrostatic latent images are developed, and, surprisingly, residual images of toner particles from the previous cycle are removed by combinations of mechanical, triboelectric and electrostatic actions of the cascading developer. It is believed that the developer particles physically knock and scrub residual toner particles from the surface of the photoconductor, and that the toner-free portions of carrier particles electrostatically attract and thereby scavenge the residual toner particles. The surprising ability of the cascade system to simultaneously remove residual images while developing electrostatic latent images is most advantageous in achieving the objects of the present invention.

A preferred embodiment of cascade development cleaning apparatus is illustrated in FIG. 2 wherein the developer 24 is shown cascading at 25 over the surface 11 of the drum within the development-cleaning zone.

Development-cleaning in the xerographic process can be enhanced by the addition of an electrode placed adjacent and parallel to the xerographic plate. The electrode may comprise a solid sheet, a screen, a series of wires, or a series of points suspended or located over or near the plate surface, said electrode being connected by conductor with a suitable potential source creating the desired electric field between the electrode and the photoconductor. The electrode is preferably biased to a voltage of the same polarity as the electrostatic image on the plate.

The preferred embodiment of a cascade developmentcleaning apparatus illustrated in FIG. 2 shows an electrode 26 adjacent to the plate surface 11. The apparatus depicted in FIG. 2 also illustrates an electroded developer control bafile 27 which is designed to control carrier flow and toner action, such as toner clouds, near the exit of the development-cleaning zone. While controlling extraneous toner, the control baffle 27 also assists development in the same manner as the primary development-cleaning electrode.

It is found that in the dynamic system of the present invention and in continuously operating automatic apparatus such as that described in FIG. I, as well as other electrostatographic apparatus embodying the advantageous system of the present invention, there are a number of variables which may directly affect the quality of operation of the inventive system.

One of those variables is the angle at which the cascading developer advances relative to the surface of the photoconductor. It is observed in both FIGS. 1 and 2 that that portion of the xerographic drum over which developer is cascaded forms an are so that a tangent to the drum surface at any point along that arc forms a different angle with the horizontal. Hence, at any point on the surface of the drum, the developer is generally passing along the photoconductor at approximately a given angle. However, because the developer is a particulate mixture, at any point along the arc of drum surface or flat plate surface in other embodiments, individual carrier beads, toner particles and combinations thereof, typically will be traveling in directions and at angles somewhat different from that made by the plate and electrode at that point. However the mean developer path will generally follow the direction and angle of the plate and electrode at a given point.

In another preferred embodiment of the invention, the developer may be cascaded over a flat plate comprising the photoconductive insulating layer on a conductive backing, and the flat plate and the accompanying electrode may be oriented at any angle with the horizontal up to The flat plate embodiment may be used in the conventional mode, that is, cascade across the surface of the photoconductor itself, or in the inverted mode, wherein the developer cascades along the electrode which is closely spaced adjacent and parallel to the surface of the photoconductor. It has been found that maximum development and cleaning occur when the xerographic plate or electrode angle is in the optimum range of about 70 conventional to about 70 inverted, from the horizontal. That is, optimum development-cleaning occurs when the plate and electrode are in a substantially vertical position. A preferred range for the angle between the electrode and the horizontal is from about 60 to about 60 inverted. Electroded, cascade development-cleaning is performed satisfactorily when the angle between the electrode and horizontal is about 20 to about 20 inverted, the lower limit being about the angle of repose of the particular system. The angle of repose is the angle formed with the horizontal by the xerographic plate or the accompanying electrode at which developer will start to flow down the surface of the uncharged plate or the surface of the electrode when operating in the inverted mode. Developer will flow over the surface which is placed at an angle somewhat less than the angle of repose if the developer is applied with an initial velocity. However, at such low angles developer flow tends to be unstable and the quality of development-cleaning is reduced. It should also be noted that although the optimum range is in a substantially vertical position or at large angles, at very low angles the conventional cascade is more efficient than the inverted cascade. Generally, maximum cleaning occurs in the area of the development-cleaning zone contiguous to the photoconductor where developer activity is greatest.

It is noted that the conditions which produce the best development-cleaning might be reproduced in a system where there is a flow of carrier accelerated by means other than or in addition to gravity, contiguous to a xerographic plate in an orientation other than that preferred in the present invention. However, such a system would probably be more complex than the advantageous system of the present invention, and therefore be less desirable from the standpoints of cost, and machine size and complexity.

Another variable is the voltage difference between voltages, development-cleaning electrode and the exposed (background) and unexposed (image) portions of the imaged plate.

The electrode voltage, V,,, must be just enough lesser in magnitude and of the same polarity as the voltage V of the unexposed image areas of the plate, to substantially enhance the field defined by the electrostatic latent image so that uniform deposition of toner will occur across such image areas. Yet, V should be of sufficient magnitude and same polarity as V, the background voltage, so that the voltage difference, A V=V,V,,, between background and exposed areas of the plate and the electrode, substantially completely inhibits toner deposition in background areas, still without adversely affecting toner deposition in image areas.

It is understood that V, and V, are necessarily dependent upon the initial charging voltage placed on the plate at the beginning of a xerographic cycle. The background voltage, V is the voltage potential remaining at exposed areas of the surface of the photoconductor after those areas have been partially discharged by light impinging upon said areas during the exposure step. This relationship is discussed at length later herein. It is found that a preferred range for the initial charging voltage is in the range of about ri-200 to about +700 volts. An optimum range of initial charging voltages is in the range of about +300 to about 500 volts.

Corresponding to the above preferred ranges, electrode voltages V,, which produce the desired and advantageous development-cleaning, are optimum in the range of about +150 to about +300 volts, and preferred in the range of about +150 to about +500 volts. lt will be appreciated that electrically negative initial charging voltages, electrostatic latent images, and development-cleaning electrode voltages may also be used. Satisfactory development-cleaning may occur at voltages above or below the indicated preferred range. However, at lower voltage magnitudes, the cleaning efi'rciency of the development-cleaning system is reduced almost linearly. While satisfactory development-cleaning is performed at voltages above the preferred range for various systems embodying the present invention, sticking of cascade carrier beads to the photoconductor may occur at such higher voltages.

Bead sticking is the adherance of carrier beads to the xerographic plate surface which results when the electrostatic attraction of the xerographic plate for the toner and the toner for the carrier bead are together greater than the mechanical forces, such as gravity, accelerating the carrier bead. When the concentration of toner in the developer is lowered, carrier beads stick to the surface of the plate, and the surface of the xerographic plate is susceptible to being scratched and pitted by the sticking carrier beads as the plate passes through closely fitted apparatus during other steps in the xerographic process.

Another variable in the novel cascade development-cleaning system of this invention is the developer flow rate. Maximum cleaning results where the developer activity is greatest. So, the greatest active residence time of developer near the plate is desired. Development-cleaning efficiency per unit time increases with increasing flow rate. As a general rule, the greater the flow rate the better the cleaning efficiency, although the developer will perform development-cleaning insofar as possible at any flow rate. The charge density of the residual image will also affect the development-cleaning efficiency and can therefore influence the selection of developer flow rate. Charge density is the charge per unit area of plate surface and not to be confused with image density which is defined later herein. On an absolute basis, images developed from higher initial charge density latent images are preferably cleaned at higher developer flow rates. However, at lower flow rates, the percent cleaned or cleaning efi'rciency is proportional to the flow rate, regardless of the initial charge density. At larger total flows (grams of developer per second per unit width of plate) images developed from plates charged to lower initial charge densities are more completely cleaned.

It has also been found that the toner particle size is a significant factor in this invention. Toner particle size affects the efficiency of the electrostatic transfer of toner to latent electrostatic images and the transfer of residual toner from the xerographic plate back to the carrier. It has been found that both processes become more efficient with larger toner particle sizes. At a given toner concentration, smaller toner particles tend to cover more of the surface of the carrier beads thereby leaving less free bead surface available for developmentcleaning or scavenging. The smaller toner particles are also less susceptible to being physically knocked from the plate surface. It has therefore been found advantageous to use toners having a particle size distribution which contains minimal amounts of relatively small toner particles. Toner particles may be classified as to particle size in a classifier for fine dry powders such as the Sharples K8 Super Classifier, manufactured by the Sharples Company, 424 West Fourth Street, Bridgeport, Pennsylvania. In the Sharples scale, toner particles are measured in microns. Toners with particles of average size by number in the range of about l0 to about 20 microns, with negligible numbers of particles of size less than 5 microns, give results preferred over those of average size in the range of about 4 to about 7 microns, with about 50 percent of the particles of a size less than 5 microns. Toners in both of the above ranges give development-cleaning efficiencies which are preferred over those attainable with particles of average size in the range of about 2 to about 3 microns, with about percent of the particles less than 5 microns in diameter. The smaller tone particles will still perform the development-cleaning, although the build up of toner-film on the apparatus typically is accelerated.

Another parameter is toner concentration in the developer mixture. The concentration of toner affects developmentcleaning primarily in the development part of the process. The cleaning will go on, but if the toner concentration is too high, the cleaned residual images will be redeveloped as quickly as they are cleaned. Hence, the limiting concentration at one end is development capability (i.e., sufficient toner to develop electrostatic latent images) while the other end point is the limit of the cleaning ability of the system. These concentration limitations depend on the degree of quality of copy desired. Toner concentration is conveniently expressed in terms of mass per unit surface area, said surface being the surface of the carrier particles or beads. The advantageous cascade development-cleaning system of the present invention produces satisfactory results in toner concentration ranges of about 0.1 to about 0.4 mg. of toner per sq. cm. of carrier surface. At toner concentrations lower than about 0.1 mg/sq.cm., development in extended unexposed areas of the image pattern still occurs, but image tone uniformity tends to fall off rapidly. At higher toner concentrations the ability to clean is reduced. The reduction in cleaning capability may in part be due to increases in the amount of residual toner retained as the residual image. It is also thought that there is increased redevelopment of the residual image at these higher toner concentrations. A preferred range of toner concentration in the developer mixture is about 0.2 to about 0.3 mg/sq.cm. These concentrations indicate that it is most desirable to closely control the toner concentration, preferably by automatic means.

Problems related to higher toner concentration include toner impaction and toner agglomeration, which may greatly reduce image quality.

It has been found that the addition of small amounts of dry solid hydrophobic lubricants effectively controls toner impaction and agglomeration. Such lubricants include metallic salts of fatty acids such as zinc searate, and other materials such as colloidal pyrogenic silica particles such as CabO-Sil", available from the Cabot Corporation, or various mixtures of such materials. An extensive group of such lubricants is recited in copending application Ser. No. 702,306, filed Feb. 2, 1968, now U.S. Pat. No. 3,552,850. A preferred range of concentrations for the lubricant is in the range of about 0.1 to about 1 percent by weight of toner.

The other component in the developer is a granular material called carrier which by mixing with the toner particles triboelectrically acquires charge of polarity opposite that acquired by the toner. Carrier granules may be any shaped solid particle from flat platelets to cubes to spherical heads. The carrier may be made of any suitable material such as glass, plastic, metal or other granular material. Carrier granules of average size in the range of about 30 to about 1,000 microns perform satisfactorily. A preferred range of carrier particle size is in the range of about to about 600 microns.

Another variable effecting development-cleaning in the inventive system, where the electrostatic latent image is formed by the preferred mode of charging and exposing a xerographic plate, is the charging voltage initially charging the photoconductor. Initial charging voltage is often also referred to as the initial potential on the photoconductor or as the initial surface potential. It is found, surprisingly, that the cleaning efficiency may be affected in portions of the plate carrying residual images by the magnitude of the initial charge. The rate of decay of charge in plate areas which are masked by residual toner images may be less than the rate of decay in unmasked areas. In the masked areas the residual toner particles may prevent light from reaching and therefore discharging those areas of 'the photoconductor during the exposure and discharge steps of subsequent xerographic cycles. Such charged residual image areas may then be redeveloped thereby increasing the amount of residual image on the plate. However, it has been found that if the initial charging voltage is kept below about +700 volts, or below about .-l-500 volts for optimum results, residual transferred images on subsequent copies are eliminated.

Furthermore, it has been found advantageous to charge the plate supporting the residual image with charge opposite in polarity to the charge of the latent image before that portion of the plate enters the charging step of the subsequent xerographic cycle. This charge will be negative, assuming the plate is initially charged positively. Charging by a negative corona charge device after transfer has proven effective in reducing the bonding forces on the toner and promoting relatively uncomplicated toner removal in the development-cleaning process. In FIG. 1 this negative charging is performed at charging station 21. Of course, it is understood that the selection of other toner and carrier materials of opposite triboelectric characteristics to those used in the above example would facilitate use of charges of opposite polarity on the elements of the system.

The initial charging voltage in the development-cleaning system is also of importance when taken in conjunction with the relative exposure illumination used during the exposure step of the xerographic process. In a xerographic apparatus, relative exposure illumination is a unit of measure of the amount of light which reaches and then discharges exposed areas of the xerographic plate. The relative exposure illumination is expressed in terms of f-number. The f-number is a number indicating the relative aperture of a particular lens or diaphragm opening in conventional projection apparatus, where relative aperture equals F/D, where F is the focal length of the lens and D the effective diameter of the aperture. Where the light source is a constant, all lenses with the same fnumber in theory form images of equal brilliance of the same subject no matter what the focal length of the lens or the diameter of the aperture. Conventional projection apparatus is typically used in the exposure step in xerographic apparatus, and it should be noted that the value for relative exposure illumination is typically constant for such embodiments of the xerographic process. However, a different embodiment may include projection apparatus with adjustable diaphragm apertures corresponding to different exposure values for which corresponding charging voltage values or ranges should be used. The values of relative exposure illumination and charging voltage are instrumental in determining the quality of the transferred image produced by xerographic apparatus embodying the advantageous system of this invention. FIG. 3 illustrates the relationship of Charging Voltage and Relative Exposure Illumination. As mentioned above in conjunction with other variables, development-cleaning may not operate satisfactorily if the charge in the background areas of the image to be developed is so strong that residual toner images cannot be adequately cleaned from said areas. In FIG. 3, line 30 represents approximate maximum charging voltages corresponding to relative exposure illuminations for a particular set of parameters, at which the system will still adequately clean residual image areas. It has also been mentioned above that the initial charging voltage put on the photoconductor must be so sufficient that the dark image areas of the electrostatic latent image as projected onto the photoconductor will be fully developed by the particular development system. Line 31 in FIG. 3 represents charging voltages corresponding to various relative exposure illumination values which are approximately minimum values of the charging voltage which will result in images which may be adequately developed to produce satisfactory quality in copies made by that particular system.

A preferred range for the initial charging voltage on the surface of the photoconductor is in the range of about +200 to about +700 volts. An optimum range of initial charging voltages is in the range of about +300 to about +500 volts. Correspondingly, a preferred range relative exposure illumination to which the surface of the photoconductor is exposed is in the range of about f/8 to about f/5.6. However, background voltages resulting from charging and exposing at values outside these ranges may produce satisfactory development-cleaning and good copy quality depending upon the particular set of parameter values which define the operating range for the given system.

Referring to FIG. 3, then, lines 30 and 31 define an envelope 32 which contains most points corresponding to particular values of the charging voltage and relative exposure illumination at which development cleaning is satisfactorily operable and copy quality is good. It is also noted that broken line 33 closes the lower portion of the envelope in FIG. 3, illustrating that when the charge on the xerographic plate is low and the relative exposure illumination projected onto the charged xerographic plate is also low, the electrostatic latent image on the plate is not sufficiently distinct to give acceptable copy quality.

This is simply another way of illustrating that the background voltage, V, and image voltage, V are such that when used in conjunction with a given electrode voltage, V,,, in a given system, the copier including the advantageous development cleaning system of the present invention will produce copies which meet or exceed commercial standards for copy quality.

It must be noted that FIG. 3 is a representative example of the approximate operating range of one development-cleaning system and that the data shown on the particular drawing of FIG. 3 are applicable only to a given set of parameters. Changes in any one or any combination of the various parameters in the development-cleaning system may vary the values which would be shown in the plot of charging voltage against relative exposure illumination.

It will be understood that an optimum embodiment of the present invention is an embodiment comprising parameter settings within the optimum ranges of each of the independent variables in the inventive system. For example, an optimum embodiment is a xerographic process including an electroded, cascade development-cleaning system wherein the photoconductor and electrode are substantially vertical to the horizontal plane, the electrode voltage is in the range of about to about +300 volts, toner size is in the range of about 10 to about 20 microns with negligible numbers of toner particles of size less than 5 microns, toner concentration in the range of about 0.2 to about 0.3 mg/sq.cm., carrier size in the range of about 250-300 microns, initial charging voltage in the range of about +300 to about +500 volts, and relative exposure illumination in the range of about f/8 to about f/5.6. In addition, various preferred embodiments may be constructed using parameter settings in any and all possible combinations of the various preferred ranges of the individual parameters in the inventive system.

In addition to the preferred cascade mode of developmentcleaning as fully described and disclosed above, other methods of development-cleaning may be used in electrostatographic processes. Another preferred embodiment of the advantageous development-cleaning system of the present invention comprises passing the electrostatic latent image support surface having said image thereon, in contact with a magnetic brush. The brush is formed from magnetic carrier particles to which toner particles are electrostatically attached. In this mode of development-cleaning, the toner particles are attached to the magnetic carrier and are then electrostatically transferred to the imaged areas of the support surface. Simultaneously, residual toner images in background areas on the support surface are removed by the electrostatic and mechanical action of the magnetic bristles of the brush. It is noted that in the background areas of the support surface, where said surface has been substantially discharged during other process steps, that the attractive force between the support surface and residual toner particles is less than the attractive forces between the brush bristles and the residual toner particles. Therefore, the magnetic brush system easily removes such residual toner particles and is a suitable specific embodiment for the advantageous development-cleaning system of the present invention. Magnetic brush systems, which have previously been used only for development of electrostatic latent images, are disclosed in Wilson U.S. Pat. No. 2,846,333, and Thompson U.S. Pat. No. 3,064,622. It is noted that the supply of toner particles in a magnetic brush system may be either a mass consisting essentially of magnetic or nonmagnetic toner particles, or a liquid suspension of magnetic or nonmagnetic toner particles.

Another specific embodiment of the advantageous development-cleaning system of the present invention comprises a fur, applicator-cleaner brush, which is located adjacent to and in contact with the electrostatic latent image supportsurface, and also adjacently contacting a supply of toner particles for the development of electrostatic latent images. In this system, the toner particles and material from which the bristles of said brush are made, triboelectrically interact so that toner particles adhere to the brush bristles, which in turn apply said toner particles to the electrostatic latent image support surface, thereby viewably developing said image. At the same time, the fur bristles triboelectrically and mechanically remove residual toner particles from essentially the same area of the support surface. A fur brush development system suitable for use as a specific embodiment of the advantageous development-cleaning system of the present invention, is disclosed in Greaves U.S. Pat. No. 2,902,974.

In still another specific embodiment of the advantageous development-cleaning system, a mass of developer is supported in contact with the electrostatic latent image support surface, and the surface supporting said image is passed through the mass of developer. As the surface passes through the mass of developer, the toner, carrying a charge opposite to the charge in the imaged areas on the support surface, adheres to said images areas thereby producing a viewable image pattern. Simultaneously in essentially the same area of the support surface, residual toner images remaining on said surface from previous electrostatographic cycles, are removed by the combination of electrostatic attraction of residual toner to the carrier particles in the developer, and by the scrubbing action of the developer on the residual toner particles. Like the preferred cascade system, this latter system has previously been found effective as a development system, but before the invention of the advantageous system of the present invention, it was not believed useful as a development and cleaning system simultaneously. Copending application Ser. No. 528,846, filed Feb. 2], 1966, now U.S. Pat. No. 3,503,776, discloses a C-shell development system which is also a specific embodiment of the present development-cleaning system comprising passing the electrostatic latent image support surface through a mass of developer particles.

Yet another specific embodiment of the advantageous development-cleaning system of the present invention comprises passing the electrostatic latent image support surface having said image thereon, through a fluidized bed of developer. A mass of developer particles may be fluidized by passing a stream of gas upwardly through the mass of developer particles thereby suspending the particles in the flowing gas stream. Alternatively, a mass of developer particles may be fluidized by mechanically vibrating the entire mass, thereby suspending some of the moving particles. As the support surface moves through this fluidized bed, the advantageous development-cleaning of the present invention is performed in the same fashion as in other specific embodiments. A fluidized bed of developer particles is disclosed in Mott U.S. Pat. No. 3,008,826,and in Donalies U.S. Pat. No. 3,393,663, and used therein solely for development, but not heretofore used for cleaning residual images from an electrostatographic surface, nor from the advantageous simultaneous development-cleaning system of the present invention.

The advantageous system of the present invention is useful in any electrostatographic process having an electrostatic latent image support surface. in the preferred process, xerography, the electrostatic latent image support surface is the surface of a photoconductive insulating layer. Selenium in its amorphous form is found to be a preferred photoconductive insulating material for use in xerography because of its extremely high quality image making capability, relatively high light response, and capability to receive and retain charged areas at different potentials and of different polarity. Any suitable photoconductive insulating layer may similarly be used in the practice of the invention. However, it is found that the inventive system performs more satisfactorily if the electrostatic latent image support surface is quite smooth. Typical photoconductive insulating layers include: amorphous selenium, alloys of sulfur arsenic or tellurium with selenium, selenium doped with materials such as thallium, cadmium sulfide, cadmium selenide, etc., particulate photoconductive materials such as zinc sulfide, zinc cadmium sulfide, French process zinc oxide, phthalocyanine, cadmium sulfide, cadmium selenide, zinc silicate, cadmium sulfoselenide, linear quinacridones, etc., dispersed in an insulating inorganic film forming binder such as a glass or an insulating organic film forming binder such as an epoxy resin, a silicone resin, an alkyd resin, a styrene-butadiene resin, a wax or the like. Other typical photoconductive insulating materials include: blends, copolymer, terpolymers, etc., of photoconductors and nonphotoconductive materials which are either copolymerizable or miscible together to form solid solutions and organic photoconductive materials of this type include: anthracene, polyvinylanthracene, anthra-quinone, oxadiazole derivatives such as 2,5-bis-(p-amino-phenyl-l 1,3,4-oxadiazole; 2-phenylbenzoxazole; and charge transfer complexes made by complexing resins such as polyvinylcarbazole, phenolaldehydes, epoxies, phenoxies, polycarbonates, etc., with Lewis acid such as tetrachlorophthalic anhydride; 2,4,7-trinitro-fiuorenone; metallic chlorides such as aluminum, zinc or ferric chlorides; 4,4-bis(dimethylamino) benzophenone; chloranil; picric acid; 1,3,5-trinitrobenzene; l-chloroanthraquinone; bromal; 4- nitrobenzaldehyde; 4-nitrophenol; acetic anhydride; maleic anhydride; boron trichloride; maleic acid, cinnamic acid; benzoic acid; tartaric acid; malonic acid and mixtures thereof.

In addition to the advantageous use of the inventive system for simultaneously developing and cleaning an electrostatic latent image support surface, it is clear that the system of the present invention may also be used as a separate cleaning system. The use of such a system for the sole purpose of cleaning would fail to achieve some of the objectives of the simultaneous development-cleaning system, such as reduction in machine size, improvement of machine cleanliness, and improvement in the life of the electrostatic latent image support surface. However, the use of the advantageous system of the present invention solely as a cleaning system, is in itself novel since no previous cleaning system using developer as the functional cleaning mechanism, has heretofore been successful in a one-pass operation.

A one-pass cleaning system using developer as the functional cleaning medium, also shows that the advantageous development-cleaning system of the present invention can be used as both a development system and a cleaning system, in any two-cycle electrostatographic process. In such two-cycle processes, the development occurs during the first cycle and the cleaning occurs during the second cycle, which cycle is solely for the purpose of removing residual toner images from the electrostatic latent image support surface. Unlike the dual station system described in the preceding paragraph, the twocycle system achieves all of the objects of the preferred inventive system, except that the recycling may involve slightly more complicated mechanisms and electrical circuits.

Although the description of the preferred embodiments of the inventive system has been primarily directed to the use of the inventive system in a xerographic process, it is appreciated and intended that the advantageous system of the present invention be incorporated in any sort of electrostatographic process.

The following examples further specifically define the present invention with respect to a system which when used in conjunction with an electrostatic latent image support surface provides for the substantially simultaneous removal of residual toner images and development of electrostatic latent images or charge patterns on essentially the same surface area. The parts and percentages are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of the development-cleaning system of this invention.

Charge density in electrostatic latent images has previously been mentioned as a factor affecting development-cleaning. However, in practice charge density on the photoconductor is not easily measured, especially in automatically recycling xerographic apparatus. But, charge density in the unexposed portion of an electrostatic latent image, above a certain lower limiting surface potential value is approximately proportional to the image density of the original copy, where image density =D=log of HR where R equals the ratio of reflected light to incident light. Standard image densities are illustrated in the Examples. For instance, in a very dense area of an original where only one-tenth of the incident light is reflected back to the eye of the viewer, R would equal onetenth and the log of HR, i.e., image density, would be I. A density of 1.3 is where about one-twentieth of the incident light is reflected back to the viewer. Densities in the range of about 1.2-1.5 or above appear to the unaided eye as a very dense black. Image densities therefore provide a convenient basis for comparing the quality of xerographic or other copies to the original from which said copies are made.

EXAMPLE I Using a 4 inches by 4 inches flat plate with selenium as the photoconductor; the plate is charged to a surface potential of about +500 volts, exposed, developed and the developed image is transferred to paper so that the average solid area image density of such transferred images is about 1.3. The plate carrying the residual image is then exposed to a 300 watt photoflood source at about l inches for about 30 seconds after each development and transfer sequence. The exposure by the photoflood source discharges the photoconductor. This cycle is then repeated many times. The developer is composed of a glass bead carrier, about 250 microns in average diameter, Xerox 914 toner, at a toner concentration of about 0.3 mg/sq.cm., a developer flow rate of about 2 g./inc. width of plate/sec. and total flows of 25, 50 and 100 g. respectively. A development electrode, a 4 inches by 4 inches stainless steel flat plate is biased at about +100 volts and placed parallel to and about 0.06 inch from the selenium surface. Developmentcleaning runs are made at various electrode angles in both the conventional and inverted modes. Data from the development-cleaning runs shown that optimum development-cleaning occurs at electrode angles near the vertical (90) with preferred cleaning in the range of about 60 conventional to about 60 inverted.

EXAMPLE II A xerographic plate and developer mix as in Example I are used. The plate is charged as in Example I and exposed to produce images with a transferred developed image density of about 1.3. A developer flow rate of about g./in.-sec., 100 grams total flow of developer is used in an inverted electroded cascade development-cleaning apparatus with the same electrode and electrode-plate spacing as in Example I at an electrode angle of 75 inverted; the electrode voltage is varied from 0 to about +1 ,200 volts. Preferred development-cleaning occurs in the range of about +200 to about +700 volts. For this particular set of parameter settings, some bead sticking is found to occur at electrode bias values above about +l,200 volts.

EXAMPLE III A standard 914 copier manufactured by the Xerox Corporation, Rochester New York, is fitted with apparatus embodying the present invention. The development-cleaning housing includes an electrode assembly having a development-cleaning electrode about 2.5 inches long (from entrance to exit measured along its are as it parallels the drum surface) and spaced about 0.08 inches from the drum surface. The drum carries an amorphous selenium photoconductor. The electrode assembly rides against the edges of the drum thereby minimizing adjustment problems. in addition, an electroded battle is introduced immediately following the developmentcleaning electrode to control excessive developer losses occuring at the exit of the development-cleaning zone. A pretransfer corotron is installed after the development-cleaning zone, before the transfer corotron, to increase transfer effi ciency. The developer mix is composed of uncoated leaded glass beads of average diameter of about 250 microns, with Xerox 914 toner, of average size in the range of about 12 to about 15 microns, and one-eighth of 1 percent by weight of the toner of zinc stearate lubricant. The electrode voltage is about +200 volts. This embodiment of the developmentcleaning system continually produces high-quality solids, halftones and lined images with low background over extended machine runs of more than 50,000 prints. Such image quality is enhanced by maintaining toner concentration at approximately 0.28 mg./sq.cm. of carrier bead surface. Toner consumption and toner efficiency for this embodiment are excellent, often greater than 15,000 8-% by 11-inch copies per pound of toner. Because the brush cleaning system is eliminated the life of the photoconductor is also increased. The cleaning station which in former copiers contributed to toner agitation and therefore was a source of toner dirt, is eliminated in the present embodiment, which is found to operate significantly more cleanly than former copiers.

EXAMPLE IV Using the copier embodying the present invention described in Example III, with an initial charging voltage of about +400 volts; relative exposure illumination of about f/6.3; toner concentration of about 0.28 mg./sq.cm. by weight; leaded glass bead carrier of average size of about 250 microns; an electrode voltage of about volts, the development-cleaning system of the present invention operates satisfactorily enabling the xerographic apparatus to produce high numbers of copies of excellent quality.

EXAMPLE V Using the apparatus embodying the present invention as described in Example III, with a transfer corotron current of about 16 microamperes, various test copies are made at different charging potentials and exposure levels. Standards are adopted for acceptable operation and copy quality: the system must clean the residual image when input image density is about 1.3 and develop that input image density to a trans ferred image density of about L2, and the system must also develop an input image density of about 0.3 to the density of a prepared test standard of density of about 0.2 Data from various runs defines an operating range approximately as shown in FIG. 3. The envelope 32 is enclosed by curve 30 representing the maximum charging potential which allows cleaning meeting the standards imposed in this Example and in the range of exposures shown, and curve 31 which represents the maximum exposure which allows development meeting the standards and in the range of charging potential shown. At lowcharging potentials and low exposure levels, the operating area is bounded by the condition that satisfactory maximum density cannot be obtained; that is, the standard of producing a transferred image density of about 1.2 from an input image density of about 1.3 cannot be met. In FIG. 3 broken line 33 represents this limitation. The envelope 32 defines an approximate operating range including those values of charging voltage and relative exposure illumination at which this apparatus and particular set of parameter values operate satisfactorily to produce copies of image density meeting the standards.

Although specific components and proportions have been stated in the above description of the preferred embodiments of the development-cleaning system, other suitable materials and variations in the various steps in the system as listed herein, may be used with satisfactory results and various degrees of quality. In addition, other materials and steps may be added to those used herein and variations may be made in the process to synergize, enhance or otherwise modify the properties of the invention. For example, various photoconductive materials may be used in xerographic plates, and various photoconductor thicknesses may require somewhat different parameter settings for preferred results.

It will be understood that various other changes in the details, materials, steps, and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention, will occur to and may be made by those skilled in the art, upon a reading of this disclosure, and such changes are intended to be included within the principle and scope of this invention.

What is claimed is:

l. A method of simultaneously developing and cleaning an electrostatic latent image support surface, comprising:

providing an electrostatic latent image support surface,

providing a first electrostatic latent image on said support surface,

developing a toner image corresponding to said first electrostatic latent image by contacting said first electrostatic latent image with toner particles, transferring said toner image from said support surface, leaving a residual toner image corresponding to said first electrostatic latent image on said support surface,

providing a second electrostatic latent image on essentially the same area of said support surface which supports said residual toner image, said second electrostatic latent image having a different image configuration from said first electrostatic latent image,

providing an electrode closely spaced adjacent the face of the support surface which supports said residual toner image, said electrode adjacent essentially the same area of said support surface which supports said residual toner image and said second electrostatic latent image, wherein said electrode is electrically biased to a potential of the same polarity and lesser magnitude than the polarity of the electrostatic latent image, and

passing developer, said developer comprising a mixture of carrier granules and toner particles, contiguous the electrostatic latent image support surface between said surface and said electrode, whereby, simultaneously, the second electrostatic latent image is developed into a toner image corresponding to said second electrostatic latent image, and said residual toner image corresponding to said first electrostatic latent image is removed from said electrostatic latent image support surface.

2. The method of claim 1 wherein said electrostatic latent image support surface is a surface of a photoconductive insulating layer.

3. The method of claim 2 wherein an electrostatic latent image is provided on said surface of said photoconductive insulating layer by steps comprising:

uniformly electrically charging said surface of said photoconductive insulating layer, and

exposing said surface with an image pattern of activating electromagnetic radiation, whereby an electrostatic latent image corresponding to said image pattern is formed on said surface.

4. The method of claim 3 wherein said photoconductive insulating layer is initially charged to a potential in the range between about +200 to about .+700 volts.

5. The method of claim 4 wherein said electrode is biased to a potential in the range between about +150 volts and about .+500 volts.

6. The method of claim 4 wherein said electrode and surface make an angle of not less than about 60" above a horizontal plane.

7. The method of claim 6 wherein the toner particles in the developer are in the range of about 10 to about 20 microns with about 1 percent or less by number of particles of size less than 5 microns.

8. The method of claim 7 wherein the concentration of toner in the developer is in the range of about 0.2 to about 0.3 milligrams per square centimeter of carrier surface.

9. The method of claim 8 wherein the carrier granules are of average size in the range of about to about 300 microns.

10. The method of claim 3 wherein said photoconductive insulating layer is initially charged to a potential in the range between about +300 to about +500 volts.

11. The method of claim 10 wherein said electrode is biased to a potential in the range between about volts and about +300 volts.

12. The method of claim 2 wherein the developer passes in contact with the electrostatic latent image support surface.

13. The method of claim 2 wherein the developer passes the electrostatic latent image support surface in a path closely spaced adjacent said surface.

14. The method of claim 2 wherein said photoconductive insulating layer comprises amorphous selenium.

15. The method of claim 1 wherein said toner image corresponding to said first electrostatic latent image is transferred from said support surface to another toner image support surface.

16. The method of claim 1 wherein said toner image corresponding to said second electrostatic latent image is transferred from said support surface to another toner image support surface, and no toner image corresponding to said first electrostatic latent image is transferred to said other toner image support surface.

17. The method of claim 1 wherein:

said electrostatic latent image support surface is a surface of a photoconductive insulating layer comprising amorphous selenium,

said electrostatic latent images are provided by electrostatically charging said support surface to a potential in the range between about .-l-200 and about +700 volts, and exposing said support surface with an image pattern of activating electromagnetic radiation thereby substantially reducing said potential in the imagewise exposed areas of said support surface,

said developer comprises carrier granules of average size in the range between about 100 and about 300 microns, toner particles of average particle size in the range between about 10 and about 20 microns with about 1 percent or less by number of said toner particles of size less than about 5 microns, and wherein the concentration of toner in said developer is in the range between about 0.2 and about 0.3 milligrams per square centimeter of carrier surface, and

said electrode is biased to a potential in the range between about +150 and about +300 volts.

18. The method of claim 1 wherein the developer passes in contact with the electrostatic latent image support surface.

19. The method of claim 1 wherein the developer passes the electrostatic latent image support surface in a path closely spaced adjacent said surface.

23. The method of claim I wherein the concentration of toner in the developer is in the range of about 0.2 to about 0.3 milligrams of toner per square centimeter of carrier surface.

24. The method of claim 23 wherein the carrier granules are of average size in the range of about I00 to about 300 microns.

25. The method of claim 20 wherein said electrode and surface make an angle of not less than about 60 above a horizontal plane.

* t #8 i i a *zgggy I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,649,262 Dated Marchl4, 1972 Inventor (s) Ronald L. Cade; Stewart William Volkers It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown be low:

Column 2, line 54', delete "particles", insert images Column 2, line 55, between "the" and "xerographic" insert surface of the Column 5, line 41, "development cleaning" should read development-cleaning Column 6., line 58, delete "voltages", insert the Column 7, line 9, 500" should read +500 Column 10, line 34, "development cleaning" should read development-cleaning Signed and sealed this 10th day of October 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents @233? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,649, 262 Dated March 14, 1972 Inventor(s) Ronald L. Cade; Stewart William Volkers It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 54, delete "particles", insert images Column 2, line 55, between "the" and "xerographic" insert surface of the Column 5, line 41, "development cleaning" should read development-cleaning Column 6., line 58, delete "voltages", insert the Column 7, line 9, 500" should read +500 Column 10, line 34, "development cleaning" should read development-cleaning Signed and sealed this 10th day of October 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

2. The method of claim 1 wherein said electrostatic latent image support surface is a surface of a photoconductive insulating layer.
 3. The method of claim 2 wherein an electrostatic latent image is provided on said surface of said photoconductive insulating layer by steps comprising: uniformly electrically charging said surface of said photoconductive insulating layer, and exposing said surface with an image pattern of activating electromagnetic radiation, whereby an electrostatic latent image corresponding to said image pattern is formed on said surface.
 4. The method of claim 3 wherein said photoconductive insulating layer is initially charged to a potential in the range between about +200 to about +700 volts.
 5. The method of claim 4 wherein said electrode is biased to a potential in the range between about +150 volts and about +500 volts.
 6. The method of claim 4 wherein said electrode and surface make an angle of not less than about 60* above a horizontal plane.
 7. The method of claim 6 wherein the toner particles in the developer are in the range of about 10 to about 20 microns with about 1 percent or less by number of particles of size less than 5 microns.
 8. The method of claim 7 wherein the concentration of toner in the developer is in the range of about 0.2 to about 0.3 milligrams per square centimeter of carrier surface.
 9. The method of claim 8 wherein the carrier granules are of average size in the range of about 100 to about 300 microns.
 10. The method of claim 3 wherein said photoconductive insulating layer is initially charged to a potential in the range between about +300 to about +500 volts.
 11. The method of claim 10 wherein said electrode is biased to a potential in the range between about +150 volts and about +300 volts.
 12. The method of claim 2 wherein the developer passes in contact with the electrostatic latent image support surface.
 13. The method of claim 2 wherein the developer passes the electrostatic latent image support surface in a path closely spaced adjacent said surface.
 14. The method of claim 2 wherein said photoconductive insulating layer comprises amorphous selenium.
 15. The method of claim 1 wherein said toner image corresponding to said first electrostatic latent image is transferred from said support surface to another toner image support surface.
 16. The method of claim 1 wherein said toner image corresponding to said second electrostatic latent image is transferred from said support surface to another toner image support surface, and no toner image corresponding to said first electrostatic latent image is transferred to said other toner image support surface.
 17. The method of claim 1 wherein: said electrostatic latent image support surface is a surface of a photoconductive insulating layer comprising amOrphous selenium, said electrostatic latent images are provided by electrostatically charging said support surface to a potential in the range between about +200 and about +700 volts, and exposing said support surface with an image pattern of activating electromagnetic radiation thereby substantially reducing said potential in the imagewise exposed areas of said support surface, said developer comprises carrier granules of average size in the range between about 100 and about 300 microns, toner particles of average particle size in the range between about 10 and about 20 microns with about 1 percent or less by number of said toner particles of size less than about 5 microns, and wherein the concentration of toner in said developer is in the range between about 0.2 and about 0.3 milligrams per square centimeter of carrier surface, and said electrode is biased to a potential in the range between about +150 and about +300 volts.
 18. The method of claim 1 wherein the developer passes in contact with the electrostatic latent image support surface.
 19. The method of claim 1 wherein the developer passes the electrostatic latent image support surface in a path closely spaced adjacent said surface.
 20. The method of claim 1 wherein said electrostatic latent image support surface and electrode are substantially vertical to a horizontal plane.
 21. The method of claim 1 wherein the toner particles in the developer are of average particle size in the range of about 10 to about 20 microns with about 1 percent or less by number of particles of size less than 5 microns.
 22. The method of claim 21 wherein the concentration of toner in the developer is in the range of about 0.2 to about 0.3 milligrams per square centimeter of carrier surface.
 23. The method of claim 1 wherein the concentration of toner in the developer is in the range of about 0.2 to about 0.3 milligrams of toner per square centimeter of carrier surface.
 24. The method of claim 23 wherein the carrier granules are of average size in the range of about 100 to about 300 microns.
 25. The method of claim 20 wherein said electrode and surface make an angle of not less than about 60* above a horizontal plane. 